Electric motors have a wide field of application. In industrial production, for example, electric motors are used to drive pumps, conveyor belts, overhead cranes, fans, etc. An electric motor, adapted for use in a specific application, offers the user many advantages, mainly owing to its long life and limited need for maintenance. One basic requirement for a long electric motor life is that the rotor in the electric motor does not have any faults or defects. Common types of rotor faults are, for example, breaks or cracks/fractures in a rotor bar, excessively high resistance in welded or soldered joints in the rotor, excessively large air cavities (as a result of the casting of the rotor) and rotor offset in air gaps relatively to the stator. Common types of stator faults are, for example, insulation faults between the turns of a winding, insulation faults between windings in the same phase, insulation faults between windings in different phases, insulation faults between windings and earth/motor casing, contaminated windings (i.e. impurities such as moisture, dust, or insulation charred due to overheating), an open turn of a winding in a delta-connected motor as well as contact problems between the winding ends and external connections.
When measuring electric three-phase motors, it is common to measure current fundamental components during operation and to compare measurement data from the three phases. Usually, special sensors are used in these measurements to obtain measurement data.
Measuring methods carried out during operation are sensitive to disturbances in the power grid, i.e. fundamentals generated by other machines (for example switched power supply units, fluorescent tube fittings, etc.) that are connected to the same power grid. These disturbances cause erroneous measuring results and may even make measurements on the electric motor impossible.
When measuring stators according to prior art, a powerful surge voltage with high energy content is supplied to the motor, following which the exponentially decaying response obtained is analysed to identify possible faults in the stator. This measuring method has many disadvantages: it is a destructive method that may initiate or accelerate/bring to completion incipient insulation failures; it requires time-consuming and complex calculations and interpretations/analyses; it causes problems of pulse propagation in the winding due to L and C effects; it requires bulky and heavy equipment associated with transport/installation problems; and it is an expensive method.